Engines for automotive vehicles may be configured and controlled to operate at varying proportions of air and fuel in their combustion mixtures. When combustion engines operate at a higher than stoichiometric air-to-fuel mass ratio, this mode of operation is referred to as “fuel-lean,” and typically results in increased fuel combustion efficiency and, thus, improved fuel economy. Compression ignition engines, such as diesel engines, traditionally and primarily operate in a fuel-lean mode. Spark ignition engines, such as gasoline engines, may also be controlled to operate in a fuel-lean mode to take advantage of the associated improved fuel economy.
Today, advanced internal combustion engines may be controlled, such as by a computer module, to operate in a plurality of modes. For example, some advanced combustion engines may selectively operate in fuel-lean and fuel-rich modes to realize better combustion stability and fuel economy, while decreasing the amount of certain combustion emissions in the exhaust gas.
The exhaust gas from an engine operating in a fuel-lean mode has a relatively low temperature and contains undesirable gaseous emissions, such as carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx). Specifically, the exhaust gas temperature from an engine operating in a fuel-lean mode may be 150 degrees Celsius, or below. Periods of even lower operating temperatures may occur, such as during the initial cold-start period of engine operation, and when the engine is running at low speeds or at idle. It is desired to treat such exhaust gas compositions to minimize the discharge of any substance to the atmosphere other than nitrogen (N2), carbon dioxide (CO2), and water. In order to convert the gaseous emissions of CO, HC, and NOx into these more innocuous gases, the exhaust gas may be passed through a treatment system where it can contact at least one catalyst to help (1) oxidize the CO to CO2, (2) oxidize the HC to CO2 and water, and (3) reduce the NOx to N2.
An exhaust gas treatment system for a combustion engine may include an oxidation catalyst for the oxidation of CO to CO2 and HC to CO2 and water. A traditional oxidation catalyst includes one or more platinum group metals (PGMs) supported as catalysts. PGM refers, collectively, to six metallic elements on the periodic table and includes ruthenium, platinum, palladium, rhodium, osmium, and iridium. However, the activation temperature of commercially-available PGM-based oxidation catalysts is typically greater than 175° C., but, as discussed above, the exhaust gas temperature from a lean-burn engine can be about 150° C., or lower.
In order to increase the oxidation performance of a traditional PGM-based oxidation catalyst at relatively low temperatures, the amount of PGM in the catalyst can be increased. However, increasing the PGM loading of the oxidation catalyst can only lower the activation temperature of the catalyst to a limited degree. And, since PGMs are relatively expensive, increasing the PGM loading can significantly increase the cost of the catalyst. There is a need for a less-expensive oxidation catalyst with equal, or preferably better, CO and HC oxidation activity at low temperatures in order to treat the exhaust gas from modern lean-burn and advanced combustion engines that have relatively low operating temperatures.